1. Field of the Invention
The present invention relates to a Bloch line memory device, and more particularly to the structure of stripe magnetic domains which stores vertical Bloch lines as information carriers.
2. Description of the Related Art
Like a magnetic bubble memory device, a Bloch line memory device uses a magnetic garnet film as a memory medium. The magnetic garnet film is formed such that its film plane has a (111) face. However, an information storing manner is greatly different between both the devices. Namely, in the magnetic bubble memory device, the presence or absence of a bubble domain corresponds to information of "1" or "0". In the Bloch line memory device, on the other hand, the presence or absence of a pair of Bloch lines in a magnetic wall enclosing a stripe magnetic domain which has a stretched magnetic domain form, corresponds to information of "1" or "0". This situation is illustrated in FIG. 1. Referring to the figure, an upward arrow 103 in a stripe magnetic domain 2 existing in a magnetic garnet film 6 represents the direction of magnetization in the stripe magnetic domain 2, and an arrow 5 in a magnetic wall 1 represents the direction of magnetization of the magnetic wall 1. Particularly, an arrow 101 on a center line in the magnetic wall 1 represents the direction of magnetization of the magnetic wall at a central portion of the magnetic wall 1. An arrow 102 perpendicular to the face of the magnetic wall 1 represents the direction of magnetization at a central portion of a vertical Bloch line (hereinafter referred to simply as Bloch line). A portion 4a where a pair of Bloch lines 3 exist corresponds to information of "1" while a portion 4b where no Bloch line exists corresponds to information of "0".
The pair 4a of Bloch lines 3 used as an information carrier is a microstructure of domain wall which exists in the magnetic wall 1 enclosing the stripe magnetic domain 2, as is shown in FIG. 1. The Bloch lines 3 exist stably in the magnetic wall 1 and can move freely in the magnetic wall 1. Therefore, if a multiplicity of the stripe magnetic domains 2 are arranged in a juxtaposing manner with the Bloch lines 3 in the magnetic walls of the respective stripe magnetic domains, a memory section like a minor loop of the magnetic bubble memory can be constructed.
The existence of such Bloch lines are known, and experiments and analyses thereof have demonstrated that the speed of movement of a magnetic domain becomes slower because of the existence of Bloch lines. In the magnetic bubble memory device in which magnetic domains must be moved, a magnetic domain including Bloch lines is called a hard bubble and efforts have been made to prevent the generation of such hard bubbles. On the other hand, in the Bloch line memory device, the existence of Bloch lines are positively utilized.
The physical dimension of a Bloch line is about one tenth of the width of a stripe magnetic domain having the Bloch line therein. Many Bloch lines can be included in one stripe magnetic domain. For example, in the case where there is used a magnetic garnet film, having been developed for use in a magnetic bubble memory, in which the width of the stripe magnetic domain is 1 .mu.m, the stripe magnetic domain can involve about 5.times.10.sup.8 Bloch lines per 1 cm.sup.2. Accordingly, when two Bloch lines are paired as an information carrier, a memory in the class of 256 Mbit/cm.sup.2 can be fabricated.
The Bloch lines have a reason other than the possession of its fine size why a memory having a large capacity can be provided. Namely, since in the Bloch line memory device a magnetic field in a direction perpendicular to a surface or film plane of the magnetic garnet film is used to propagate or transfer information though in the magnetic bubble memory device an information carrier is propagated by rotating an in-plane field, a propagation track pattern in the Bloch line memory device assumes a simplified planar configuration and construction, which facilitates an improvement in density of the device.
FIG. 2 shows an example of the propagation track pattern. A Bloch line pair 4 exists in a magnetic wall enclosing a stripe magnetic domain 2 which is present in a magnetic garnet film 6 formed on a substrate 9. The stripe magnetic domain 2 exists to enclose a grooving 8 formed in a part of the magnetic garnet film 6. The grooving 8 is one means for arranging many stripe magnetic domains 2 in juxtaposition relation with each other and fixing their positions. The Bloch line pair 4 is moved in the magnetic wall 1 by a magnetic field H.sub.P applied perpendicularly to the surface of the magnetic garnet film 6. In order that the Bloch line pair 4 is moved by a fixed amount at this time, a propagation track pattern 7 is provided on the stripe magnetic domain 2. The Bloch line pair 4 is fixed by an in-plane field 104 having the same direction as the magnetization 105 of a region enclosed by the Bloch line pair. By selecting the magnitudes of the perpendicular and in-plane fields H.sub.P and 104 so as to be within their predetermined ranges and applying the perpendicular field H.sub.P once, the Bloch line pair 4 can be moved below the next propagation track pattern 7. The requirement of the propagation track pattern 7 for realizing this operation is only to have a function of generating the in-plane field 104 and hence the pattern 7 may be provided by the repetition of a simple rectangular shape, as shown in FIG. 2.
As apparent from the above explanation, the Bloch line pair as an information carrier can freely move around the stripe magnetic domain, In order to write and read information, as required, it is necessary to dispose function sections at the outside of the memory section. A Bloch line memory is schematically shown in FIG. 3. The memory section is constructed of a multiplicity of juxtaposed stripe magnetic domains 2. The propagation track pattern 7 is provided crossing at an approximately right angle with respect to the stripe magnetic domains 2. Read and write function sections 121 and 120 are provided at the right and left sides of the stripe magnetic domains 2, respectively.
The writing of Bloch lines is effected by causing a current to flow through a conductor formed in the vicinity of a left end portion of the stripe magnetic domain 2 to generate a localized magnetic field so that the magnetization of the magnetic wall is inverted by 180.degree.. Namely, the magnetization of the region 4b of "0" shown in FIG. 1 is inverted so as to take the direction of magnetization of the portion 4a of "1". At this time, the magnetization continuously changes at an interface between the inverted region and a non-inverted region to produce a state in which the magnetization is changed by 90.degree. with respect to the magnetic wall. This state yields a Bloch line. Such Bloch lines are necessarily produced in a paired form and hence a memory is constructed by corresponding a pair of Bloch lines to one unit of information.
When it is desired to write "0" or no Bloch line pair in the stripe magnetic domain, a distance between the left end portion of the stripe magnetic domain 2 and the writing conductor is made large so that a magnetic field causing the inversion of magnetization does not act on the stripe magnetic domain 2. In order to distance only a desired stripe magnetic domain from the writing conductor, means is used which causes a magnetic bubble 10 to exist at the end portion of that stripe magnetic domain. The magnetic bubble 10 causes an electromagnetic repulsive force to act on the left end portion of the stripe magnetic domain 2, thereby contracting the stripe magnetic domain 2. As a result, the above-mentioned positional relation is realized between the writing conductor and the stripe magnetic domain 2. In order to cause magnetic bubbles to exist in the write function section 120, the write function section 120 is provided with a magnetic bubble transfer track 110 and a magnetic bubble generator 11. The magnetic bubble propagation track 110 transfers a magnetic bubble 10 from the magnetic bubble generator 11 to the left side of a predetermined stripe magnetic domain 2.
The reading of information is effected by converting the presence or absence of Bloch lines into the presence or absence of a magnetic bubble. For this conversion is used a method described by Konishi on pages 1838-1840 and 1841-1843 of IEEE TRANSACTIONS ON MAGNETICS, vol. Mag-19, No. 5 (1983). Namely, when a Bloch line is present, the directions of magnetization of the magnetic wall on the opposite sides of the Bloch line are inverted with respect to each other. Because of this change of the structure of domain wall, a change takes place with respect to the easiness of chopping of an end portion of the stripe magnetic domain when one Bloch line is moved to the end portion of the stripe magnetic domain. By selecting a predetermined chopping condition, therefore, the chopping of a magnetic bubble from the stripe magnetic domain can be made only in the case where one Bloch line is present at the end portion of the stripe magnetic domain. This chopping is carried out at the read function section 121. The chopped magnetic bubble is transferred through a propagation track 111 for read-out to a magnetic bubble detector 12 by which the presence of the magnetic bubble is converted into an electric signal. The presence or absence of this electric signal corresponds to the presence or absence of the magnetic bubble and hence the presence or absence of the Bloch lines (i.e. information) at the end portion of a predetermined stripe magnetic domain. In this way, the reading operation is carried out. Such a Bloch line memory device is described in detail by U.S. Pat. No. 4,583,200.
The above-described Bloch line memory device has a problem that information or Bloch line is trapped at the memory section or function section(s) so that the information is lost or deviates from a desired address position, thereby making accurate reading of information impossible. This problem will be explained by use of FIG. 4. FIG. 4 is a view showing the state of a stripe magnetic domain 2 existing in a magnetic garnet film 6 formed on a substrate 9. A Bloch line pair 4 is present in a magnetic wall 1 of the stripe magnetic domain 2. A conductor 14 is provided for controlling the position of the Bloch line pair 4. The conductor 4 assumes one of conductors included in the function sections 120 and 121 shown in FIG. 3. As apparent from FIG. 3, the conductor 14 is arranged crossing a longitudinal direction of the stripe magnetic domain 2 at an approximately right angle.
The conductor 14 is formed or worked by photolithography. Usually, since the thermal expansion coefficient of the conductor 14 is different from that of the magnetic garnet film 6, a residual stress remains after the formation of the conductor 14. This stress gives a change to a perpendicular magnetic anisotropy of the magnetic garnet film 6. Namely, a magnetic easy axis of the magnetic garnet film 6 is usually in a direction perpendicular to the film plane but the residual stress produced by the conductor 14 changes the magnetic easy axis just below the conductor 14 to a direction in the film plane. Therefore, there is exhibited a phenomenon that the magnetization 103' which is upward at the outside of the stripe magnetic domain 2 falls into the film plane at a portion 150 just below the conductor 14. This phenomenon takes place also in the magnetic wall 1 in which Bloch lines exist. When the falling of the magnetization into the film plane takes place, this in-plane magnetization acts as a resistance in moving a pair of Bloch lines. As a result, the Bloch lines are trapped by the in-plane magnetization, thereby making smooth propagation of the Bloch lines impossible.
A similar problem takes place also just below the propagation track pattern 7. This problem is common to all the patterns formed on the magnetic garnet film.